1. Field of the Invention
The present invention relates to a diffractive optical element for use in an optical system of a high-resolution imaging system, and a method of manufacturing the diffractive optical element.
2. Description of the Related Art
With the recent increase in density of integrated circuits, a projection exposure apparatus, which is used for manufacturing a semiconductor device, needs to project a circuit pattern formed on a reticle onto a wafer by exposure with higher resolution. For that purpose, the numerical aperture of a projection optical system has been increased, and the number of lenses that make up the optical system has also been increased. In addition, there has been a demand for a larger screen. As a result, the projection exposure apparatus has increased in size and cost.
Furthermore, the wavelength of exposure light has been shortened from the spectral i-line that is a bright-line spectrum of Hg, to 248 nm of laser light to be emitted from a KrF excimer laser or an ArF excimer laser, and to 193 nm of ultraviolet light. This limits the available lens materials, and therefore makes it difficult to correct chromatic aberration of the optical system. In order to solve this problem, an attempt has recently been made to incorporate a binary optical element, which is a blazed transmissive grating formed of a concentric grating on a plane-parallel plate, into the optical system.
A general-type of optical system is basically composed of spherical lenses. In the case of a spherical convex lens, the focal length increases as the wavelength increases. On the other hand, since the binary optical element having a similar light bending action to that of the lens utilizes the diffraction of light, its focal length decreases as the wavelength increases. Consequently, the incorporation of the binary optical element in the optical system makes it possible to easily correct chromatic aberration and to reduce the number of lenses.
In incorporating the above-mentioned binary optical element in the optical system, however, it is difficult to precisely adjust the optical axis. In a conventional lens system, a mechanical centering method and an optical centering method are adopted to adjust the optical axis. In the mechanical centering method, a lens 1 coated with oil is held by a clamp 2 from both sides, and the clamp 2 is turned, as shown in FIG. 10. Thereby, the lens 1 is shifted so that the turn axis and the optical axis of the lens 1 align with each other, as shown in FIG. 11. This method is not applicable to the binary optical element because it is planar on both sides and is not shifted even when the clamp is turned.
In the optical centering method, a lens 1 is bonded to a turn jig 3 and turned as shown in FIG. 12, and the turn axis is shifted while viewing a crosshairs chart 4 and a reference mark chart 5 so that images of both the charts align with each other, thereby centering the optical axis of the lens 1. In this case, there is a need to previously align the optical axis of the binary optical element with the center of the turn jig 3, and therefore, precise adjustment of the optical axis is impractical.
Accordingly, it is an object of the present invention to provide a diffractive optical element whose optical axis can be positioned with high precision, and a method of manufacturing the diffractive optical element.
In order to achieve the above object, according to one aspect, the present invention provides a diffractive optical element including a region provided with a concentric uneven pattern and having a predetermined light bending characteristic, and a peripheral section worked so that the center of the profile of the diffractive optical element aligns with the optical axis of the diffractive optical element determined by the light bending characteristic of the region.
According to another aspect, the present invention provides a diffractive optical element manufacturing method including a first step of making a diffractive optical element provided with a concentric uneven pattern thereon, and a second step of working the periphery of the diffractive optical element so that the optical axis and the center of the profile of the diffractive optical element align with each other.
According to a further aspect, the present invention provides a diffractive optical element manufacturing apparatus including turning means for turning a diffractive optical element about a predetermined turn axis, detection means for detecting the offset between the turn axis and the optical axis of the diffractive optical element, position adjustment means for adjusting the relative positions of the turn axis and the diffractive optical element based on the detection result of the detection means, and cutting means for cutting the periphery of the diffractive optical element.
In addition, optical equipment using the diffractive optical element of the present invention, in particular, a projection exposure apparatus, and a method of manufacturing a device by utilizing the projection exposure apparatus will be disclosed in the following description of the preferred embodiments of the present invention.